The Advent of Flexible Coal

03/01/2014 | Sonal Patel

The increasing penetration of intermittent renewable generation, smart grids, demand response, and other emerging technologies has underscored the need for power plants with greater flexibility and efficiency—and one surprising solution could come from new and existing coal plants, suggests a new study from the U.S. National Renewable Energy Laboratory and Intertek for 21st Century Power Partnership.

Coal plants, says the report, “Flexible Coal: Evolution from Baseload to Peaking Plant,” though widely perceived to provide only baseload generation, can be modified to cycle on and off and run at lower output (below 40% of capacity). The document details a demonstration to increase flexibility at a North American coal generating station—which is unnamed for “commercial reasons”—a feat that requires “limited hardware modifications but extensive modifications to operational practice,” it claims.

“Cycling does damage the plant and impact its life expectancy compared to baseload operations. Nevertheless, strategic modifications, proactive inspections, and training programs, among other operational changes to accommodate cycling, can minimize the extent of damage and optimize the cost of maintenance,” it says.

According to the report, the plant was originally intended to run as a baseload unit at an 80% annual capacity factor when it came online in the 1970s, but it has “at times cycled on and off as many as four times a day to meet morning and afternoon peak demand.” The authors add that “The overarching impact of this type of cycling is thermal fatigue but also stresses on components and turbine shells resulting from changing pressures, wear and tear on auxiliary equipment used during cycling, and corrosion caused by oxygen entering the system and condensation from cooling steam.” Those consequences of cycling can take several years to show up as damage or forced outages.

Several physical modifications were made to the boilers, pulverizers, turbines, rotors, and condensers at the plant, but once the physical changes were in place, “90% of future savings in costs came from adjustments to operating procedures,” the report reveals. For example, establishing procedures and training to control boiler ramp rates has been especially effective, as have been high-energy piping inspections.

The report echoes several conclusions reached by a number of prominent analytical entities, and it likewise suggests that if modified to be more flexible, older coal units can still serve a purpose in an increasingly low-carbon energy system.

Most coal power plants are “capable of some dynamic operation and are designed to be able to cycle with moderate ramp rates and potentially even handle two-shift operation (where the plant is started up and shut down daily),” observes the International Energy Agency’s Coal Industry Advisory Board (CIAB) in a 2013 report titled “21st Century Coal: Advanced Technology and Global Energy Solution.” However, the increased need for flexibility “will impact costs, maintenance, and reliability,” the CIAB also concludes.

“Most notably, higher cycling will increase wear and tear while the number of operating hours decreases, resulting in an increase of specific maintenance costs/MW-hr over time. Moreover, as coal power plants add more complex environmental control systems such as [carbon capture and storage] in the future, their ability to operate dynamically may be reduced,” the CIAB says.

Yet, as illustrated by some countries with a high share of intermittent renewables, if a portfolio of strategies involving both technical and operational improvements is implemented, the flexibility of current and future coal plants can be achieved, the CIAB suggests.

One prominent example is Germany, which is moving to produce 80% of its power with renewables by 2050. A tenfold increase in wind and solar photovoltaic capacity in Germany since 2000 has resulted in a second “feed in” load fluctuation in addition to the traditional consumer demand fluctuation.

Meanwhile, in a much-cited paradox for the country that is promoting a massive shift to renewables with billions of euros in subsidies, Germany’s production of coal-fired power rose in 2013 to its highest level since 1990 as natural gas prices soared. Last November, Steag opened its 725-MW Walsum-10 unit near the western city of Dortmund, and Trianel started commercial operation of a 750-MW Lünen plant (Figure 2) in North Rhine-Westphalia in December. Meanwhile, eight hard coal power plants are scheduled to begin operation in the next two years, including Vattenfall’s 1.5-GW Moorburg plant near Hamburg and RWE’s Hamm facility in the Dortmund area.

2. Hard but flexible coal. The 750-MW Lünen hard coal–fired power plant owned by Trianel Kohlkraftwerk Lünen came online in December 2013 in northwest Germany and is predicted to run 7,000 full-load operating hours in 2014. Siemens Energy and IHI Corp., which built the turnkey plant, say it has an efficiency of almost 46%. The plant’s Siemens SST5-6000 steam turbine is designed to enable highly responsive ramping, which is crucial to meeting load adjustments posed by intermittent renewable generation. Courtesy: Siemens Energy

According to the CIAB, Germany’s existing power plants are optimized “to cater to flexible operation,” even if they were built before expansion targets for wind and photovoltaic plants had been adopted. “In many plants, measures to allow greater flexibility have been implemented subsequently, so that power plants can meet increased requirements for market load adjustments. As a result, there are very few baseload plants that do not allow for flexible operation, it notes. At the same time, new coal-fired power plants are specifically designed for flexible operation. “Pure baseload power plants are no longer being built.”

The CIAB notes, however, that Germany also suffers higher electricity prices than most developed countries. That means the impact of increased costs due to the fluctuating operation of conventional power plants is “somewhat less significant,” it says.

—Sonal Patel, associate editor(@POWERmagazine, @sonalcpatel)

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